Generally, an image forming apparatus has a toner cartridge removably mounted for reserving a toner. When the toner residue in a toner cartridge becomes less than a predetermined quantity, for example, the toner may not be sufficiently transferred to the recorded medium thereby to cause the so-called “faint printing”. When the toner in the toner cartridge is degraded (in the charging performance), it may not be transferred to the proper position on the recorded medium thereby to cause the so-called “printing fog”.
In view of the above problem, there is disclosed in JP-A-2001-337568 an image forming apparatus for deciding the timing for exchanging the toner cartridge. Specifically, the image forming apparatus measures the lapse of time for the toner supply from the instant when a new toner cartridge was mounted in the image forming apparatus body, and inhibits the recording operation (or the image forming operation) by assuming the arrival of the exchanging time.
The old cartridge having been removed for the arrival of the exchanging time may be stored in the office at the same place of a new toner cartridge till it is discarded, and may be mounted again in another image forming apparatus body of the same kind. In the image forming apparatus disclosed in JP-A-2001-337568, the exchanging time is determined on the basis of the lapse of the toner supply time from the instant when the old toner cartridge was newly mounted. Therefore, the recording operation may be permitted to cause the insufficient image formation highly probably. In short, the configuration disclosed in JP-A-2001-337568 can decide the exchanging timing properly, in case the new toner cartridge is mounted, but cannot in case the toner cartridge mounted was once used. As a result, the recording operation is executed with the used toner cartridge thereby to cause the poor image formation.